The measurement of blood pH, as part of the suite of measurements known as ABG or arterial blood gases, is considered a critical parameter in assessing the respiratory and metabolic status of the critical care patient. The pH measurement is important for diagnosing the underlying etiology of patient acid-base imbalances. It is also critical for monitoring critical care patients who are either candidates for or are receiving ventilatory assistance.
Current approaches for measuring blood pH require removal of a blood sample from the patient. The sample is typically analyzed using a clinical blood gas analyzer that employees standard potentiometric methods. Clinical blood gas analyzers are complex instruments that require frequent maintenance in the form of cleaning and calibration, and are subject to periods of inoperability due to formation of blood clots in the sample path and/or sensor calibration failure.
Because acid-base status can change rapidly in the critically ill patient, it is often desirable to obtain frequent blood gas measurements to monitor the patient. In many cases it is desirable to obtain continuous or near-continuous information regarding patient acid-base status. However, the invasiveness of acquiring blood samples as well as the progressive blood loss associated with blood sample collection limits the frequency with which the measurement can be made.
A noninvasive method for measuring blood pH would offer numerous improvements over existing methods. It would allow for continuous measurement of a patient's acid-base status, eliminate patient blood loss associated with blood pH measurement, reduce exposure of medical staff to biohazardous material, and reduce the patient pain and morbidity associated with blood draws. It has been demonstrated that blood pH can be measured using near-infrared spectroscopy. See, e.g., M. K. Alam, J. E. Franke, T. M. Niemczyk, J. D. Maynard, M. R. Rohrscheib, M. R. Robinson, R. P. Eaton, AppI. Spec. 52, 393 (1998); M. Alam, M. Rohrscheib, J. Franke, T. Niemczyk, J. Maynard, M. Robinson, Appl. Spec. 53, 316 (1999), each of which is incorporated be reference. Accuracy of the measurement can be important to the proper care of the patient, and previous spectroscopic measurements have not conclusively demonstrated sufficient accuracy.
Accordingly, there is a need for improvements in methods and apparatuses useful in making spectroscopic measurements of blood pH.